Extended range concentric cell base station

ABSTRACT

The present invention is an extended range concentric cell base station and a method for extending a cell size or access range without incurring ASIC correlator re-design. This is accomplished with a concentric cell base station design that incorporates multiple timing protocols and search windows. The concentric base station has associated a micro cell and a macro cell, wherein the micro and macro cells use a same frequency band but different timing protocols and search windows that will cause signals transmitted by mobile-telephones within their respective cells to be received within the confines of at least one search window.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationssystems and, in particular, to extending access ranges of wirelesscommunications systems.

BACKGROUND OF THE INVENTION

FIG. 1 depicts a wireless communications system 10 employing CodeDivision Multiple Access (CDMA) techniques based on the well-known IS-95standard of the Telecommunication Industrial Association. The wirelesscommunications system 10 comprises a mobile switching center (MSC) 12and a plurality of base stations (BS) 14-i connected to the MSC 12. Eachof BS 14-i provides wireless communications services tomobile-telephones (MT), such as mobile-telephones 16-k, within anassociated geographical coverage area referred to herein as cell 18-iwith a radius R_(i). For illustrative purposes, cells 18-i are depictedas circular in shape with base stations 14-i centrally positioned. Itshould be understood that cells 18-i may also be non-circular in shape(e.g., hexagonal) with the base stations positioned non-centrally, andthat the term “radius R_(i)” should be construed to define a distancebetween the base station and a point on the circumference of cell 18-i(which will vary depending on the particular point on thecircumference).

Each base station 14-i includes radios and antennas for modulating andtransmitting base station signals to mobile-telephones, and forreceiving and demodulating mobile-telephone signals frommobile-telephones within its associated cell 18-i. Each base station14-i further includes a receiver for receiving timing information usingthe well-known Global Positioning Satellites (hereinafter referred as a“GPS receiver”).

Signals are transmitted by base stations 14-i and mobile-telephones inaccordance with a timing protocol aligned with GPS time using the GPSreceiver. FIG. 2 depicts a timing schedule 20 incorporating animplementation of a timing protocol based on the IS-95 standard. Thetiming schedule 20 comprises a series of frames 22-n, wherein each frame22-n spans a time interval t. The beginning of each frame 22-n is markedby a frame boundary at time T_(n) aligned to GPS time. In accordancewith the timing protocol, base stations 14-i are configured to begintransmitting base station signals at the frame boundaries, wherein thebase station signals include zero or more information bearing signalsand a pilot signal for coherent demodulation of the information bearingsignals by the mobile-telephones and system access operations. Bycontrast, mobile-telephones 16-k are configured to begin transmittingmobile-telephones signals at some multiple x of a frame time period(i.e., tx) after mobile-telephones 16-k began receiving base stationsignals, where x is some integer greater than or equal to zero. Unlikebase station signals, mobile-telephone signals include one or moreinformation bearing signals and no pilot signal, and are encoded using aset of orthogonal codes (referred to as Walsh codes) combined with apseudo-noise (PN) sequence (or a known code) such that the informationbearing signal may be non-coherently demodulated. The PN sequencecomprises random 0 and 1 digital signals, wherein the duration for a 0or 1 to transmit is referred to herein as a PN chip.

The above described timing protocol will now be discussed in referenceto FIG. 3, which depicts a time chart 28 illustrating a sequence oftransmissions and receptions by base station 14-i and mobile-telephone16-k. At time T₁, BS 14-i begins transmitting base station signal S₁ toMT 16-k, which may be located anywhere in cell 18-i. MT 16-k beginsreceiving signal S₁ at time T₁+d_(BS→MT), where d_(BS→MT) is apropagation delay from BS 14-i to MT 16-k. Note that the termpropagation delay should be construed to include line-of-sight andnon-line-of-sight propagation delays.

MT 16-k will wait a time interval tx from when MT 16-k began receivingsignal S₁ before it begins transmitting mobile-telephone signal S₂.Thus, MT 16-k will begin transmitting signal S₂ at time T₁+d_(BS→MT)+tx(or time d_(BS→MT) after some frame boundary). For example, if x=2, thenMT 16-k transmits signal S₂ at time T₃+d_(BS→MT) (or two frames afterreceiving the base station signal S₁).

Due to a propagation delay d_(MT→BS) from MT 16-k to BS 14-i, BS 14-iwill begin receiving signal S₂ at time T₁+d_(BS→MT)+tx+d_(MT→BS). Forease of discussion, it is assumed that the propagation delay d_(MT→BS)from MT 16-k to BS 14-i is the same as the propagation delay d_(BS→MT),and both will hereinafter be referred to individually as a one waypropagation delay d_(ow), i.e., d_(ow)=D_(MT→BS)=d_(BS→MT), orcollectively as a round trip propagation delay 2d_(ow). Thus, BS 14-iwill begin receiving signal S₂ at time T₁+tx+2d_(ow).

In order to demodulate the received signal S₂, BS 14-i must first detectsignal S₂. Each radio includes a correlator, which is a device thatdetects mobile-telephone signals. For example, the correlator detectsmobile-telephone signal S₂ by multiplying an incoming signal by the PNsequence, where the PN sequence is time shifted in discrete steps over aperiod or time interval (referred to herein as a search window W_(n))until the resulting product (of the PN sequence and the incoming signal)exceeds a threshold indicating the detection of mobile-telephone signalS₂. If BS 14-i does not begin to receive signal S₂ within the confinesof a search window W_(n), BS 14-i will not be able to detect signal S₂(using the timing protocol incorporated in FIG. 2).

To ensure that BS 14-i begins receiving signal S₂ within the confines ofsearch windows W_(n), search windows W_(n) should span time intervalsthat include possible arrival times for signal S₂ (traveling a straightline or line-of-sight path between the mobile-telephone and the basestation) regardless of the position of mobile-telephone 16-k in cell18-i. Based on the above described timing protocol, base station 14-ican expect to receive signal S₂ no earlier than the frame boundary andno later than time 2d_(ow-radius) after the frame boundary, whered_(ow-radius) is the one way propagation delay (or 2d_(ow-radius) is theround trip propagation delay) for a signal traveling a distance equal tothe radius R_(i). Thus, search windows W_(n) should span a duration ofat least 2 d_(ow-radius) beginning at time T_(n) and ending no earlierthan time T_(n)+2 d_(ow-radius). In effect, the duration of searchwindows W_(n) restricts the effective radius (or size) of cell 18-i,which is also referred to herein as the access range of a base station.

The duration of search windows W_(n) depends on the implementation ofthe correlator. Typically, correlators are implemented in the form of anApplication Specific Integrated Circuit (hereinafter referred to as an“ASIC correlator”) having a predetermined number of bits (also referredto herein as a “bit limitation”) for representing a round trip delay (ofa signal traveling from the base station to the mobile-telephone andback to the base station). Such bit limitation limits the duration ofthe search windows which, as discussed above, limits the effective sizeof cell 18-i or access range of the base station 14-i. As long as thebit limitation does not limit search windows W_(n) to a duration of lessthan 2 d_(ow-radius), base station 14-i should be able to detect signalS₂ transmitted by any mobile-telephone located anywhere within its cell18-i (assuming that R_(i) is the same for all points on thecircumference).

Typical implementations of base stations in an IS-95 based CDMA wirelesscommunications system include an ASIC correlator having a 12-bitlimitation for representing the round trip delay. In order to have fineresolution of delay, a typical value of 1/8 PN chip is used as theminimum resolution unit. The 12-bit limitation (or round trip delayrepresentation) in units of 1/8 PN chips yields a range of 512 PN chips(i.e., 2¹² bits×1/8 PN chips/bits). For a transmit bandwidth of 1.2288MHz (which is typical for an IS-95 based CDMA wireless communicationssystem), the 12-bit limitation can represent a round trip delay of 416μs (i.e., 512 PN chips÷1.2288 PN chips/μs). With air propagation speedof 5.33 μs/mile, the 416 μs round trip delay (or 208 μs one way delay)represents the limitation that if a mobile-telephone is locatedapproximately 39 miles (i.e., 208 μs÷5.33 μs/mile) from the basestation, the mobile-telephone is capable of communicating with the basestation if the radio path loss is acceptable and the search window isconfigured correctly—that is, the 12-bit limitation (or 512 time chipdelay index representation) allows for a cell with a maximum radiusR_(i) (or a maximum round trip delay) of approximately 39 miles. Asignal transmitted by a mobile-telephone beyond 39 miles of BS 14-i, inaccordance with the prior art timing protocol, may not arrive at BS 14-iwithin the confines of any search windows W_(n) and, thus, will not bereliably detectable with the 12-bit ASIC correlator.

Presently, if the cell size or access range is to be extended beyond the12-bit limitation of the ASIC correlator (i.e., beyond 39 miles), theASIC correlator would have to be re-designed. Specifically, the ASICcorrelator would have to be re-designed to increase its bit limitationsuch that signals transmitted by mobile-telephones positioned beyond theaccess range 12-bit limitation of the ASIC correlator may also bedetected. ASIC correlator re-design, however, is undesirable and may notbe economical for small scale of applications. Therefore, there exist aneed to extend the cell size or access range of the base station withoutincurring the high costs associated with ASIC correlator re-design.

SUMMARY OF THE INVENTION

The present invention is an extended range concentric cell base stationand a method for extending a cell size or access range without incurringASIC correlator re-design. This is accomplished with a concentric cellbase station design that incorporates multiple timing protocols andsearch windows. The concentric base station has associated a micro celland a macro cell, wherein the micro and macro cells use a same frequencyband but different timing protocols and search windows that will causesignals transmitted by mobile-telephones within their respective cellsto be received within the confines of at least one search window. In oneembodiment, the micro cell uses the timing protocol of the prior artwith a first search window that begins at the frame boundary and ends atsome time p₁ after the frame boundary, wherein p₁ represents a timeinterval corresponding to a bit limitation of an ASIC correlator beingused to represent the first search window. By contrast, the macro celluses a modified timing protocol and a second search window that beginsafter the frame boundary but no later than the time p₁ after the frameboundary (i.e., no later than the end of the first search window) andends at some time p₂ after the second search window began, wherein themodified timing protocol will cause the signals transmitted bymobile-telephones in the macro cell to be received within the confinesof the second search window and p₂ represents a time intervalcorresponding to a bit limitation of an ASIC correlator being used torepresent the second search window.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, aspects, and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 depicts a wireless communications system employing Code DivisionMultiple Access (CDMA) techniques based on the well-known IS-95standard;

FIG. 2 depicts a timing schedule used in accordance with oneimplementation of a timing protocol based on the IS-95 standard;

FIG. 3 depicts a time chart illustrating a sequence of transmissions andreceptions by base station and mobile-telephone in accordance with thetiming schedule of FIG. 2;

FIG. 4 depicts a base station based on the well-known IS-95 standard forCode Division Multiple Access used in accordance with the presentinvention;

FIG. 5 depicts a timing schedule for a timing protocol used inaccordance with one embodiment of the present invention;

FIG. 6 depicts a time chart illustrating a sequence of transmissions andreceptions by a base station and a mobile-telephone located within anextension of a cell;

FIG. 7 depicts a base station having a hierarchical cell structure usedin accordance with the present invention;

FIG. 8 depicts a timing schedule incorporating a first and a secondtiming protocol used by the base station of FIG. 7; and

FIG. 9 depicts a base station with a micro cell and a macro cell,wherein the micro and macro cells both have an inner and an outerradius.

DETAILED DESCRIPTION

FIG. 4 depicts a base station 30 based on the well-known IS-95 standardfor Code Division Multiple Access used in accordance with the presentinvention. Base station 30 includes radios and antennas for modulatingand transmitting base station signals to mobile-telephones and forreceiving and demodulating mobile-telephone signals frommobile-telephones within cell 34, and a GPS receiver for receivingtiming information using the well-known Global Positioning Satellites.Each radio includes a correlator implemented in the form of an ASIC(hereinafter referred to as an “ASIC correlator”) operable to detectmobile-telephone signals such that the mobile-telephone signals may bedemodulated.

For purposes of discussion, the ASIC correlator has a 12-bit limitation(or 512 PN chips) for representing a round trip delay (of a signaltraveling from base station 30 to a mobile-telephone and back to basestation 30), as described in the background section. This should not beconstrued to limited the present invention to ASIC correlators with12-bit limitations. It will be clear to one of ordinary skill in the artthat the present invention is equally applicable to base stations havingASIC correlators with other bit limitations or correlators implementedin a form other than an ASIC. A 12-bit (or 512 PN chips) ASIC correlatorhas a search window W_(n) of approximately 416 μs in duration. In priorart CDMA wireless communications systems using a timing protocol basedon the IS-95 standard, such search window W_(n) is configured to beginat time F_(n) (marking the beginning of frames) and end at timeF_(n)+416 μs, and would allow base station 30 to detect a signaltransmitted from mobile-telephones located within approximately 39 milesof base station 30. Thus, a mobile-telephone beyond 39 miles of basestation 30 would be considered beyond the access range of base station30 equipped with a 12-bit ASIC correlator.

Cell 34 has an outer radius R_(outer) (or R₃₄) and an inner radiusR_(inner) (or R₃₂), wherein outer radius R_(outer) may be or is adistance beyond the access range of the ASIC correlator bit limitation(e.g., R_(outer)>39 miles for an ASIC correlator with a 12-bitlimitation), inner radius R_(inner) is less than R_(outer), and thedifference ΔR between radii R_(outer) and R_(inner) should be no greaterthan the distance (or maximum round trip delay) corresponding to theASIC correlator bit limitation (e.g., ΔR<39 miles). Thus, part of cell34 may be beyond the access range of the ASIC correlator bit limitationin accordance with the subject invention.

The present invention allows base station 30 to detect signalstransmitted from mobile-telephones located anywhere in cell 34,including beyond the access range of its ASIC correlator bit limitation(e.g., beyond 39 miles), without ASIC correlator re-design. The presentinvention is accomplished using a modified timing protocol that willcause search windows to shift with respect to frame boundaries, therebycausing signals transmitted by mobile-telephones positioned beyond thebit limitation of the ASIC correlator to be received within the searchwindows. This involves transmitting a base station signal at a time rrelative to frame boundaries and configuring search windows W_(n) tobegin and end at a time q and q+p, respectively, after the time r,wherein q is a timing advance value greater than zero representing apropagation delay corresponding to a signal traveling no more than roundtrip between the base station and the inner radius of cell 34 (i.e., qcorresponds to a propagation delay for a distance greater than zero butno more than twice inner radius R_(inner)) and p represents a timeinterval corresponding to the ASIC correlator bit limitation or a timeinterval over which a mobile-telephone signal may be correlated andthereby detected.

In one embodiment of the present invention, base station 30 is operableto detect signals transmitted from mobile-telephone 38 using a modifiedtiming protocol incorporating shifted or offset search windows W_(n).FIG. 5 illustrates a timing schedule 70 for a timing protocol used inaccordance with this embodiment of the present invention. In accordancewith the timing schedule 70, base station 30 is configured to begintransmitting signals at the frame boundaries, and search formobile-telephone signals within shifted search windows W_(n) spanningfrom time F_(n)+q and ending no later than time F_(n)+q+p. Likewise,mobile-telephone 38 is configured to begin transmitting signals at somemultiple x of a frame time interval (i.e., fx) after themobile-telephones begin receiving base station signals. Like timingschedule 50, base station 30 using timing schedule 70 will begin toreceive signals transmitted from mobile-telephone 38 within the(shifted) search window W_(n).

FIG. 6 depicts a time chart 60 illustrating a sequence of transmissionsand receptions in accordance with the timing protocol of FIG. 5 by basestation 30 and mobile-telephone 38, which may be located anywhere withincell 34. Base station 30 begins transmitting base station signal S₁ attime F₁. Transmission of signal S₁ at time F_(n) by base station 30 willresult in the reception of signals transmitted by mobile-telephones 38within shifted search windows W_(n) such that the mobile-telephonesignals may be detected and demodulated by base station 30notwithstanding that mobile-telephone 38 is beyond the access range ofthe ASIC correlator bit limitation.

Mobile-telephone 38 begins receiving signal S₁ at time F₁+d_(ow), whered_(ow) is the one way propagation delay from base station 30 tomobile-telephone 38 (or from mobile-telephone 38 to base station 30).Since mobile-telephone 38 is in cell 34, the propagation delay d_(ow)should correspond to a time necessary for a signal to travel at least adistance R_(inner). Note that for ease of discussion, the propagationdelay from base station 30 to mobile-telephone 38 is assumed to beidentical to the propagation delay from mobile-telephone 38 to basestation 30. If mobile-telephone 38 transmits a mobile-telephone signalS₂ to base station 30, mobile-telephone 38 waits some multiple of aframe time interval (i.e., fx) from when mobile-telephone 38 beganreceiving signal S₁ before it begins transmitting signal S₂. Thus,mobile-telephone 38 will begin transmitting signal S₂ at some timeF₁+d_(ow)+fx (or time d_(ow) after some frame boundary). Because of thepropagation delay d_(ow) from mobile-telephone 38 to base station 30,base station 30 will begin receiving signal S₂ at some timeF₁+d_(ow)+fx+d_(ow) (or F₁+2 d_(ow)+fx). Since 2 d_(ow) corresponds to atime necessary for a signal traveling at least round trip between thebase station and a distance R_(inner), the signals should be positionedto be received between time F_(n) (i.e., the s frame boundary) and timeF_(n)+p, where p=416 μs corresponding to the ASIC correlator bitlimitation (or within the confines of search windows W_(n)). Signal S₂is then detected and processed using techniques well-known in the art.

It should be noted that a base station incorporating only the modifiedtiming protocol of the present invention may not be able to detectmobile-telephone signals transmitted by mobile-telephones located withincell 32. To be capable of detecting such mobile-telephone signals, thepresent invention uses a timing protocol different from the timingprotocol being used to communicate with mobile-telephones located withincell 34, as will be described herein.

FIG. 7 depicts a base station 80 having a hierarchical cell structureused in accordance with the present invention. Base station 80 hasassociated a micro cell 82 and a macro cell 84. Micro cell 82 has amicro cell radius R_(micro) or R₈₂, wherein micro cell radius R_(micro)is less than or equal to a distance corresponding to the ASIC correlatorbit limitation (e.g., R_(micro)≦39 miles). Macro cell 84 has an outermacro cell radius R_(macro-outer) or R₈₄ and an inner macro cell radiusR_(macro-inner) or R₈₅, wherein inner macro cell radius R_(macro-inner)is greater than zero and less than or equal to R_(micro), and thedifference ΔR between the macro cell radii R_(macro-outer) andR_(macro-inner) should be no greater than the distance corresponding tothe ASIC correlator bit limitation (e.g., ΔR≦39 miles for a 12 bit ASICcorrelator). Although FIG. 8 shows micro cell 82 and macro cell 84 astwo distinct cells, it should be understood that micro cell 82 and macrocell 84 may also partially overlap.

Base station 80 comprises a plurality of radios 90, one or more antennas92 and a GPS receiver 94. Each of the plurality of radios 90 areoperable to modulate and demodulate signals using a same frequency bandfreq, which includes an uplink and a downlink frequency channel. Eachradio 90 includes at least one correlator 96 implemented in the form ofan ASIC. Antennas 92 are operable to transmit and receive signals usingthe frequency band freq. Base station 80 (or radios 90) is configured totransmit signals using frequency band freq such that mobile-telephoneslocated within micro and macro cells 82, 84 receive pilot signals(transmitted by base station 80) with an acceptable signal strength.

Base station 80 provides wireless communications services tomobile-telephones, such as mobile-telephone 86, in micro cell 82 usingthe frequency band freq and a first timing protocol. The first timingprotocol, in one embodiment, is the timing protocol currently beingemployed in IS-95 based CDMA wireless communications system, asdescribed earlier in the Background section. By contrast, base station80 provides wireless communications services to mobile-telephones, suchas mobile-telephone 88, in macro cell 84 using a second timing protocolbut the same frequency band. The second timing protocol, in oneembodiment, is the aforementioned modified timing protocol depicted inFIG. 5. In this embodiment, the first timing protocol has associated afirst search window W_(1−n) that begins at frame boundaries and ends atsome time p₁ after the frame boundaries, wherein p₁ represents the timeinterval corresponding to the bit limitation of an ASIC correlator beingused to represent the first search window W_(1−n). The second timingprotocol has associated a second search window W_(2−n) that begins afterthe frame boundary but no later than the time p₁ after the frameboundaries and ends at some time p₂ after the second search windowW_(2−n) began, wherein p₂ represents the time interval corresponding tothe bit limitation of an ASIC correlator being used to represent thesecond search window.

A timing schedule 100 for the first and second timing protocols is shownin FIG. 8, in accordance with one embodiment. The timing schedule 100includes a series of frames 102-n, wherein each frame 102-n spans a timeinterval f, and the beginning of each frame 102-n is marked by a frameboundary at time F_(n) aligned with GPS time using the GPS receiver 94.In accordance with the first and second timing protocols, base station80 is configured to begin transmitting base station signals using thefrequency band freq at the frame boundaries, and search formobile-telephone signals using the frequency band freq within firstsearch windows W_(1−n) spanning from time F_(n) and ending no later thantime F_(n)+p₁. In accordance with the second timing protocol, basestation 80 is configured to begin transmitting base station signalsusing the same frequency band freq at the frame boundaries, and searchfor mobile-telephone signals using the frequency band freq within secondsearch windows W_(2−n) which begins after the frame boundary but nolater than the time p₁ and ends some time p₂ after the second searchwindow began. For purposes of illustration, the second search windowW_(2−n) is shown as beginning when the first search window W_(1−n) ends.This should not be construed to limit the present invention in anymanner.

Regardless of the timing protocol, mobile-telephones 86, 88 areconfigured to begin transmitting signals at some multiple x of a frametime interval (i.e., fx) after the mobile-telephones began receivingbase station signals, where x is some integer greater or equal to zero.When signals arrive at base station 80, base station 80 will search theincoming signals for the presence of mobile-telephone signals using bothsearch windows W_(1−n) and W_(2−n). If the signal was transmitted by amobile-telephone in micro cell 82, then base station 80 should detectthe mobile-telephone signal within the first search window W_(1−n). Ifthe signal was transmitted by a mobile-telephone in macro cell 84, thenbase station 80 should detect the mobile-telephone signal within thesecond search window W_(2−n).

In one embodiment, base station 80 uses different radios to search eachsearch windows W_(1−n), W_(2−n). In another embodiment, base station 80uses one radio to search both search windows W_(1−n), W_(2−n). In yetanother embodiment, base station 80 would not search the second searchwindow W_(2−n) for mobile-telephone signals unless base station 80 didnot detect any mobile-telephone signals in the first search windowW_(1−n).

Although the present invention has been described in considerable detailwith reference to certain embodiments, other versions are possible. Forexample, the present invention is also applicable to base stations witha micro cell and a macro cell having inner and outer radii, see FIG. 9,and wireless communication systems employing other types of multipleaccess techniques, such as time division multiple access. Therefore, thespirit and scope of the present invention should not be limited to thedescription of the embodiments

I claim:
 1. A method for detecting a mobile-telephone signal comprisingthe steps of: transmitting a base station signal at a first frameboundary; searching at the base station for a mobile-telephone signalusing a first search window beginning at a second frame boundary andending at a time p₁ after the second frame boundary, wherein p₁represents a time interval corresponding to a bit limitation for a firstcorrelator; and searching at the base station for the mobile-telephonesignal using a second search window beginning after the second frameboundary but no later than the time p₁ after the second frame boundaryand ending at a time p₂ after the second search window began, wherein p₂represents a time interval corresponding to a bit limitation for asecond correlator.
 2. The method of claim 1 further comprising the stepof: detecting the mobile-telephone signal using a correlator.
 3. Themethod of claim 2, wherein the mobile-telephone signal is detected whena resulting signal of an incoming signal multiplied with a known codeexceeds a threshold.
 4. The method of claim 3, wherein the known code isa pseudo-random noise sequence.
 5. The method of claim 1, wherein thefirst frame boundary and the second frame boundary are identical.
 6. Themethod of claim 1, wherein the first frame boundary and the second frameboundary are not identical.
 7. A method for detecting a mobile-telephonesignal comprising the steps of: transmitting a base station signal at afirst frame boundary; searching at the base station for amobile-telephone signal using a first search window beginning at asecond frame boundary and ending at a time p₁ after the second frameboundary, wherein p₁represents a time interval corresponding to a bitlimitation for a first correlator; and searching at the base station forthe mobile-telephone signal using a second search window beginning afterthe second frame boundary but no later than the time p₁ after the secondframe boundary and ending at a time p₂ after the secone seach windowbegan, wherein p₂ represents a time interval corresponding to a bitlimitation for a second correlator, wherein the step of searching forthe mobile-telephone signal using the second search window is performedonly if the nobil-telephone signal is not detected by the step ofsearching for the mobile-telephone signal using the first search window.8. The method of claim 1, wherein the first correlator and secondcorrelator are the same.
 9. The method of claim 1, wherein the firstcorrelator and second correlator are not the same.
 10. A base stationcomprising: a first radio for transmitting base station signals at aframe boundary, the first radio having a first correlator configured tosearch for mobile-telephone signal during a first time intervalbeginning at the frame boundary and ending a time p₁ after the frameboundary, wherein p₁ represents a time interval corresponding to a bitlimitation for the first correlator; and a second radio co-located withthe first radio for transmitting base station signals at a time rrelative to the frame boundary, the second radio having a secondcorrelator configured to search for the mobile-telephone signal during asecond time interval beginning after the frame boundary but to laterthan time p₁ after the frame boundary and ending at a time p₂ after thesecond time interval began, wherein p₂ represents a time intervalcorresponding to a bit limitation for the second correlator.
 11. Thebase station of claim 10, wherein the first correlator detects themobile-telephone signals when a resulting signal of an incoming signalmultiplied with a known code exceeds a threshold value.
 12. The basestation of claim 10, wherein the second correlator detects themobile-telephone signals when a resulting signal of an incoming signalmultiplied with a known code exceeds a threshold value.
 13. The basestation of claim 10 further comprising: a GPS receiver for receivingtiming information for aligning the frame boundaries.